"Balance of Power" remains one of the most important episodes of Star Trek: The Original Series. After all, this was the tale that first introduced us to the Romulans. In the adventure, a Federation outpost located in the Romulan Neutral Zone comes under attack by a Romulan warship.
When filming began on "Balance of Power," producer Bob Justman placed a call to his ace up the sleeve — special effects designer Wah Chang. The uncredited artist was the brilliant creative mind behind the communicator, the Salt Vampire, the Tribbles and other iconic Trek costumes and props. Justman asked Chang to fabricate a new alien ship with a twist. He wanted something like a bird swooping down upon its enemy to wipe them out.
Chang did just that, decorating the underbelly of his Romulan Bird of Prey with a graphic fit for the hood of a vintage Pontiac Firebird. The model was put into action and became a vital part of the standout episode.
However, in a subsequent season, when another script called for the model to be pulled out of mothballs, a tragic fate had taken the bird from us.
Wah Chang was a non-union contractor. The Bird of Prey prop was returned to him after the production of "Balance of Power." Thinking it was a one-off use, Chang had disposed of the ship. After much back and forth, producers came to the revelation and determined the budget could not afford to rebuild the prop.
In the season two episode "The Deadly Years," the Enterprise again encounters some Romulan Bird of Prey ships. To create the space scenes, stock footage was used. The following season, our heroes again face off against the Romulans in "The Enterprise Incident." However, Spock notes an immediate difference: "Intelligence reports Romulans now using Klingon design."
Yep, the Romulans were now flying Klingon spaceships. This was not some handshake deal made in the Star Trek universe. The original prop was gone and this was simply the easiest solution.
Perhaps this is why, later on, the Klingon Empire stole the term "Bird Of Prey" for their own ship, seen first in the motion pictures and later in the modern series.
The one saving grace is the AMT model company, with it's huge investment in classic Star Trek ships. The hobby giant sorted out this ship and preserved it as a classic model kit, which more recently was changed to at last make it true to the filming miniature. We recommond not destroying your own models, of course.